the discharge in the electroscope was then due to the γ rays alone. The electroscope is very advantageous for measurements of this character, and accurate observations can be made simply and readily.
The curve of decay of activity, measured by the α rays, for an exposure of 1 minute in the presence of the radium emanation is shown in Fig. 86, curve BB.
The curve exhibits three stages:—
(1) A rapid decay in the course of 15 minutes to less than 10 per cent. of the value immediately after removal;
(2) A period of 30 minutes in which the activity varies very little;
(3) A gradual decrease almost to zero.
The initial drop